In addition to the gunman, authorities said 12 people were killed and about 12 others injured when at least one shooter opened fire in Building 197 at the Navy Yard.
Located in the heart of the nation's capital, it's home to high-level naval personnel. About 3,000 people work at the Yard, including junior service members and civilian employees.
It's supposed to be a secure facility. So what went wrong?
"It'll be interesting to see as this develops who the shooter is, how he got in, those type of questions answered. I think right now a lot of people are wondering, you know, just how safe the building is, or how safe the office environment is," Jirus said.
The FBI has identified the suspected shooter as Aaron Alexis, a 34-year-old military contractor from Texas.
A contractor ID removed from the suspect's body matched the appearance of the dead man, a law enforcement source said.
Police were initially looking for two other people in connection with the shooting, but one was subsequently cleared. The other may still be on the loose.
Jirus described how access typically works.
"I have a CAC card," he said, referring to a common access card, the standard ID for active duty uniformed service personnel, Selected Reserve, Defense Department civilian employees and eligible contractor personnel.
The ID also is the main card used to grant physical access to buildings and other controlled spaces.
"As a military person, we all have badges that we have to key the door to get in -- that allows us automatic access to the building.
"But again, if you're a contractor, if you're coming to visit the building, you can go through security and walk right in -- not walk right in, but you would check in through security," Jirus said.
Even to drive or walk onto the base, a person would be required to present credentials, said Navy Capt. Mark Vandroff. Building 197 has armed security at the door.
"There's a kiosk you go through, and it either gives you a green or a red light, but the green light shows that your credentials are recognized as someone who is supposed to be in that building.
"So you have to go through a couple of layers of security to get into Building 197 -- first to get access onto the Yard, and then through the doors to get in," Vandroff said.
CNN Pentagon Correspondent Barbara Starr has visited the Navy Yard several times over the years for stories.
In her experience, security is extensive. Everyone must have clearance to be there or valid identification that allows them entry.
If you were an employee of the facility, you would have a parking sticker and go through a checkpoint, Starr said. A guard would see your sticker, know that you were authorized to be on the compound and allow you to proceed.
If you were a visitor, as Starr would be, you'd call ahead and have an appointment. Guards would have your name at the gate, and you would be escorted to wherever you needed to go.
"So the question, of course, is -- how did somebody get a weapon onto the base?" she asked.
"Look, it is always possible in these types of situations, theoretically, to find your way around security measures. Somebody could have had a weapon in their car, unseen, hidden and taken it out.
"There's any number of ways it might have happened," Starr said.